AC potential imposed to the iron electrode with the thin passive oxide induces modulation of both charge and reflectance. The modulated reflectance, i.e., potential modulation reflectance (PMR) was measured to characterize the passive oxide on iron in pH 8.4 borate buffer solution. The dependence of PMR signal on anodic potential exhibited similar behaviour of modulated charge, i.e., film capacitance. The both reciprocals of PMR signal and the capacitance proportionally increased with the thickness which linearly grew with increase of anodic potential. The Mott-Schottky type plot held for the PMR during the decreasing potential as well as the film capacitance. The spectroscopic measurement of the PMR, the light absorption edge of the passive oxide was estimated to be 2.4-2.5 eV. (C) 2014 Elsevier Ltd. All rights reserved.